1. Field of the Invention
The present invention relates to an optical deflector capable of being fabricated by techniques of micromechanics or the like.
2. Description of the Related Background Art
In recent years, development of a microactuator, using techniques of micromechanics has been energetically advanced. Electrostatic driving, piezoelectric driving, electromagnetic driving and the like can be employed as its driving means. A small-sized large-output (i.e., a relatively large deflection angle can be obtained by a relatively small electrical power) microactuator is increasingly required as demands for reduction in its cost, its application to mobile equipment, and the like increase. Particularly, an optical deflector for deflecting a movable mirror supported with a couple of torsion bars has been increasingly developed as a device capable of achieving a relatively simple optical sensor, an image forming apparatus, or the like.
As a method of driving the above-discussed movable mirror, Japanese Examined Patent Application Publication (KOKOKU) No. 60 (1985)-57051 proposes a method of deflecting or displacing a movable mirror 101 about a rotational axis (defined by a couple of torsion bars 104) by an electrostatic force acting between a couple of flat opposed electrodes 103 and the movable mirror 101, as illustrated in FIGS. 1A and 1B. Further, Japanese Patent Application Laid-Open (KOKAI) No. 4 (1992)-343318 proposes a method in which a pair of interdigitated comb electrodes 206 are formed on a movable mirror 201, another pair of interdigitated comb electrodes 207 are provided so as to mesh with the pair of interdigitated comb electrodes 206 with a gap therebetween, and the movable mirror 201 is deflected about a rotational axis (defined by a couple of torsion bars 204) by an electrostatic force acting between the two pairs of interdigitated comb electrodes 206 and 207, as illustrated in FIGS. 2A and 2B.
On the other hand, Japanese Examined Patent Application Publication (KOKOKU) No. 60 (1985)-57052 proposes a so-called moving coil method in which a coil 306 is provided on a movable mirror 301, plural permanent magnets 307 are disposed near the coil 306, and the movable mirror 301 is deflected about a rotational axis (defined by a couple of torsion bars 304) by a magnetic field generated by a current flow through the coil 306 and acting on the permanent magnets 307, as illustrated in FIGS. 3A and 3B. Further, Japanese Patent Application Laid-Open (KOKAI No. 6 (1994)-82711 proposes a so-called moving magnet method in which a permanent magnet 407 is provided on a movable mirror 401, a coil 406, is disposed near the magnet 407, and the movable mirror 401 is deflected about a rotational axis (defined by a couple of torsion bars 404) by a magnetic field generated by a current flow through the coil 406 and acting on the permanent magnet 407, as illustrated in FIGS. 4A and 4B.
Furthermore, Japanese Patent Application Laid-Open (KOKAI) No. 60(1985)-107017 proposes an actuator in which a deflector or a movable mirror, as discussed above, is provided in a rotatable manner about two rotational axes, as illustrated in FIG. 5. In the actuator, a movable mirror 501 is supported by a gimbal 505 (i.e., a movable substrate or frame) using a couple of torsion bars 504. The gimbal 505 is supported by a substrate using another couple of torsion bars 504, and the rotational axis of the movable mirror 501 is perpendicular to that of the gimbal 505. Movable portions (i.e., the movable mirror 501 and the gimbal 505) are driven by electrostatic forces from two pairs of flat opposed electrodes 503, respectively. There is also a disclosure that the movable portions can be driven by magnetic means. In such a structure, resonance frequencies of the movable mirror 501 and the gimbal 505 can be set to predetermined values, respectively, and a two-axis optical deflector capable of forming an image by raster scanning can be achieved.
In the event that the drives about two rotational axes are carried out in an electrostatic manner in the above-discussed actuator having a gimbal structure, two sets of electrodes are disposed for the drives about two rotational axes, respectively. Therefore, the problem of crosstalk between electric fields generated by the two sets of electrodes is likely to occur, irrespective of the type of the electrodes (i.e., flat opposed electrodes, interdigitated comb electrodes, or the like). Similarly, in the event that the drives about two rotational axes are carried out in an electromagnetic manner, two sets of permanent magnets and two sets of electromagnetic coils are disposed for the drives about two rotational axes, respectively. Accordingly, problems of restriction of arrangement of the magnets and crosstalk between magnetic fields generated by the two sets of permanent magnets and electromagnetic coils are likely to occur, irrespective of the type of the electromagnetic driving (i.e., the moving magnet type, the moving coil type, or the like).
Further, in a case where one drive is carried out in an electrostatic manner using the flat opposed electrode, one of power generated for driving the movable portion and the displacement amount of the movable portion must be restricted owing to a structural disadvantage of the flat opposed electrode. In other words, if an inter-electrode distance (a distance between the flat opposed electrodes) is enlarged to achieve a large displacement amount of the movable portion, a driving voltage for achieving a desired displacement angle of the movable portion must be unavoidably increased.